This invention relates to novel glass-ceramics and, more particularly, to glass-ceramics suitable for use as a light filter and, more particularly, to glass-ceramics suitable for use as a band-pass filter and a gain flattening filter.
There are light filters which cut or pass light of a specific wavelength and there are also light filters which reduce intensity of light without depending upon wavelength. The former includes a band-pass filter which passes only a specific wavelength, a notch pass filter which cuts a specific wavelength and high-pass and low-pass filters which pass only wavelengths shorter or longer than a specific wavelength. The latter includes an ND filter.
Light filters can be classified also into an absorption type filter and an interference type filter. A representative absorption type filter is the ND filter and a representative interference type filter is the band-pass filter. A substrate made of plastic is used for absorption type filters such as those for photography. Since a substrate for light filters which are subject to a strong laser beam requires durability and heat resistance property, amorphous glass is exclusively employed for such substrate.
The band-pass filters are made by forming, on a substrate made of, e.g., glass, a multi-layer film of dielectric by alternately laminating an H layer of a dielectric thin film having a high refractive index and an L layer of a dielectric thin film having a low refractive index.
In a band-pass filter which is used for the WDM(wavelength division multiplexing) optical communication system, temperature stability of the center wavelength of the band poses a problem when a narrow band width for passing wavelengths is set for applying the band-pass filter to a wavelength of a higher density. More specifically, the band-pass filter is a sensitive element in which the center wavelength of the band varies even with a slight variation in temperature and, therefore, temperature compensation should be made by a temperature controller when the band-pass filter is used. Such temperature controller, however, cannot actually be employed because of limitation in the space where the band-pass filter is located. The temperature stability has become a matter of increasing importance since it is necessary to reduce the band width as the amount of light information increases.
In the past, amorphous glass has been used as a substrate for the band-pass filter. This prior art substrate is not sufficient in its compressive stress to the film and its durability since its thermal expansion property and mechanical strength are not sufficiently high. Further, amorphous glass has low mechanical strength and therefore tends to produce micro-cracks in processing with resulting cracking or chipping off of corner portions of the product which reduces the yield of the product. Moreover, in amorphous glass, a relatively large amount of alkali ingredient must be added if a high thermal expansion property is to be provided and this poses a problem of elution of alkali ingredient during and after forming of the dielectric film on the substrate. Thus, amorphous glass cannot sufficiently satisfy the demands for a substrate for a light filter, particularly a substrate for a band-pass filter.
Known in the art are some glass-ceramics. For example, the glass-ceramics of a SiO2xe2x80x94Li2Oxe2x80x94MgOxe2x80x94P2O5 system disclosed in U.S. Pat. No. 5,626,935 containing lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) as main crystal phases is an excellent material as a material textured over the entire surface in which, by controlling the grain diameter of globular crystal grains of xcex1-quartz, the conventional mechanical texturing or chemical texturing can be omitted and the surface roughness after polishing (Ra) can be controlled within a range from 15 xc3x85 to 50 xc3x85. In this glass-ceramic, however, no discussion or suggestion is made about Young""s modulus and a coefficient for thermal expansion which are important features of the present invention.
Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a magnetic disk substrate made of a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x80x94Li2O system having predominant crystal phases of lithium disilicate (Li2O.2SiO2) and xcex2-spodumene (Li2O.Al2O3.4SiO2) which has a negative coefficient of thermal expansion. This glass-ceramic has a composition which contains a relatively large amount of Al2O3 ingredient and in which growth of SiO2 crystals such as xcex1-quartz (xcex1-SiO2) is extremely restricted and, therefore, it is difficult in this glass-ceramic to obtain a coefficient of thermal expansion required in the present invention and, moreover, since the glass-ceramic is so hard that it has no good processability. Further, since this glass-ceramic requires a high temperature of 820xc2x0 C. to 920xc2x0 C. for crystallization which prevents a large scale production of the product at a competitive cost.
International Publication WO97/01164 which includes the above described Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a glass-ceramic for a magnetic disk in which the lower limit of the Al2O3 ingredient is lowered and temperature for crystallization is reduced (680xc2x0 C.-770xc2x0 C.). A sufficient improvement however cannot be achieved by merely lowering the lower limit of the Al2O3 ingredient. Besides, crystals grown in all examples disclosed are xcex2-eucriptite (Li2O.Al2O3.2SiO2) which has a negative coefficient of thermal expansion and, therefore, has the same disadvantage as the above described prior art glass-ceramic.
It is, therefore, an object of the invention to provide a material suitable for a substrate for a light filter which has eliminated the above described disadvantages of the prior art substrate and has a thermal expansion property which is sufficient for avoiding variation in the refractive index at a temperature at which a filter formed with a mono-layer or multi-layer film is used (i.e., having a high coefficient of thermal expansion and thereby imparting compressive stress to the film to improve temperature stability of the refractive index of the film) and also has a mechanical property which imparts sufficient durability to the filter and further has excellent light transmittance.
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described object of the invention have resulted in the finding, which has led to the present invention, that, glass-ceramics having, as their predominant crystal phases, lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) or xcex1-quartz solid solution (xcex1SiO2 solid solution) and having Young""s modulus (GPa) of 95 to 120 have an excellent processability and is suitable for use as a substrate for a light filter and, more particularly, as a substrate for a band-pass filter or a gain flattening filter.
For achieving the object of the invention, there is provided a glass-ceramic having Young""s modulus (GPa) within a range from 95 to 120 and comprising 5.3 to less than 10 weight percent (expressed on oxide basis) of Al2O3.
In one aspect of the invention, the glass-ceramic has specific gravity within a range from 2.4 to 2.6.
In another aspect of the invention, the glass-ceramic has a coefficient of thermal expansion which is within a range from 65xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C. within a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C. 
In another aspect of the invention, predominant crystal phases the glass-ceramic are (a) lithium disilicate (Li2O.2SiO2) and (b) at least one of xcex1-quartz (xcex1-SiO2). and xcex1-quartz solid solution (xcex1-SiO2 solid solution).
In another aspect of the invention, the glass-ceramic is substantially free of Na2O and PbO.
In another aspect of the invention, the glass-ceramic comprises 0.3 weight percent or over (expressed on the basis of composition of the oxide) of MgO.
In another aspect of the invention, the glass-ceramic has a composition which consists in weight percent expressed on the basis of composition of oxides:
SiO2 71-81%
Li2O 8-11%
K2O 0-3%
MgO 0.3-2%
ZnO 0-1%
P2O5 1-3%
ZrO2 0.5-5%
TiO2 0-3%
Al2O3 5.3-8%
Sb2O3 0.1-0.5%
SnO2 0-5%
MoO3 0-3%
NiO 0-2%
CoO 0-3%
Cr2O3 0-3%
and has, as predominant crystal phases, a) lithium disilicate (Li2O.2SiO2) and b) at least one of xcex1-quartz (xcex1-SiO2) and xcex1-quartz solid solution (xcex1-SiO2 solid solution).
In another aspect of the invention, the glass-ceramic has, as its predominant crystal phases, lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) which have fine globular crystal grains.
In another aspect of the invention, average grain diameter of the crystal phases is 0.30 xcexcm or below.
In another aspect of the invention, the glass-ceramic is obtained by melting glass materials, forming molten glass, annealing formed glass and then heat treating the formed glass for nucleation under nucleation temperature within a range from 550xc2x0 C. to 650xc2x0 C. for one to twelve hours and further heat treating the formed glass for crystallization under crystallization temperature within a range from 680xc2x0 C. to 800xc2x0 C. for one to twelve hours.
In another aspect of the invention, there is provided a glass-ceramic substrate for a light filter using a glass-ceramic as described above.
In another aspect of the invention, there is provided a light filter provided by forming a multi-layer film on a glass-ceramic as described above.
These and other objects and features of the invention will become more apparent from the description made below.
Reasons for limiting the physical properties, surface characteristics, predominant crystal phases and crystal grain diameter, and composition will now be described. The composition of the glass-ceramic is expressed in weight percent on the basis of composition of oxides as in their base glass.
Description will be first made about Young""s modulus. As described above, as a glass-ceramic used for a light filter which is formed with a multi-layer film thereon, particularly for a band-pass filter or a gain flattening filter, it is preferable for the glass-ceramic to have the Young""s modulus as defined in the claims of the present application from the view point of processing and various handling processes. For the use as the light filter, the glass-ceramic is processed to small chips each having a size in the order of, for example, 1 mmxc3x971 mmxc3x971 mm and, if Young""s modulus is lower than the above defined range, micro-cracking or chipping off of corner portions of these small chips will take place in processing of the glass-ceramic to such small chips with resulting significant drop in the yield of the product. Micro-cracking or chipping off of corner portions of small chips does not take place at a significant rate in processing of the glass-ceramic of the present invention presumably by virtue of synergistic effect of a large Young""s modulus and restriction of growth of micro-cracks by precipitated crystal grains of the glass-ceramic.
As regards specific gravity, the glass-ceramic should preferably have as low specific gravity as possible. In most cases where the glass-ceramic is used as a light filter, many small chips, each constituting a light filter are mounted on one unit of optical fiber. The light filter made of the glass-ceramic of the present invention has excellent stability in the center wavelength of the filter band and also has high wavelength resolution and, therefore, the unit of optical fiber can receive many wavelengths of light. Accordingly, it is important to reduce the weight of the unit and, for this purpose, specific gravity of the glass-ceramic must be taken into consideration. If, however, the specific gravity of the glass-ceramic is reduced to an excessive degree, it becomes difficult to achieve a desired Young""s modulus by reason of balance of ratio between precipitated crystal phases and ratio of precipitation of crystal phases in the glass-ceramics. Having regard to such balance, it has been found that the specific gravity should preferably be within a range from 2.4 to 2.6. Having further regard to this balance, it has been found that Young""s modulus (GPa)/specific gravity should preferably be 37 or over and 50 or below.
Coefficient of thermal expansion is a very important factor for improving the wavelength resolution of the multi-layer film. More specifically, stability of center wavelength of a band against temperature is very important and, for this purpose, a coefficient of thermal expansion which is larger than that of a film forming material is required. As a result of studies and experiments made by the inventors of the present invention, it has been found that, in a band-pass filter, stability of the center wavelength against temperature depends to some extent on a refractive index temperature coefficient of a dielectric which constitutes the thin film and, to a larger extent than that, on a coefficient of thermal expansion of the substrate. This is because refractive index is also determined by a film atomic density of the thin film. That is, the higher the film atomic density of the thin film is, the smaller becomes variation caused by the temperature of the center wavelength. The film atomic density of the thin film is greatly influenced by the coefficient of thermal expansion of the substrate for the light filter on which the thin film is formed. More specifically, the temperature of the substrate during the film forming process becomes about 200xc2x0 C. and the substrate thereby is considerably expanded. The thin film is formed on this expanded substrate and, as the substrate is cooled, the thin film is subjected to compressive stress due to difference in the coefficient of thermal expansion between them. As a result, the film atomic density of the thin film increases and the refractive index thereby increases. As the coefficient of thermal expansion of the substrate increases, the compressive stress applied to the dielectric thin film formed on the substrate increases with the result that variation in the refractive index due to temperature at which the filter is used increases. In a region of the compressive stress above a certain value, variation of refractive index relative to change in temperature is saturated with a small value of variation. In other words, by imparting compressive stress above a certain value to the dielectric thin film, variation in the center wavelength relative to the temperature becomes constant with a small value of variation. For this reason, it is desirable to set the coefficient of thermal expansion of the glass-ceramic at a larger value than the coefficient of thermal expansion of the dielectric thin film.
It has been found that, if the coefficient of thermal expansion within the temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C. is 65xc3x9710xe2x88x927/xc2x0 C. or over, sufficient compression stress can be imparted to the film with a temperature range in which the glass-ceramic is used as a band-pass filter and that, if the coefficient of thermal expansion exceeds 140xc3x9710xe2x88x927/xc2x0 C. differences in the coefficient of thermal expansion between the substrate and the filter becomes so large that problems such as separation of the film from the substrate take place. A preferable range of the coefficient of thermal expansion is 65xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C., a more preferable range is 75xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C. and the most preferable range is 95xc3x9710xe2x88x927/xc2x0 C. to 130xc3x9710xe2x88x927/xc2x0 C.
As regards the shape and grain diameter of the precipitated crystal phases, crystal grains and their shape are important factors for achieving the above described characteristics of the glass-ceramic. The desired coefficient of thermal expansion cannot be achieved if the grain diameter of the crystal grains of the respective crystal phases is above or below the claimed range. The crystal grains should preferably be fine globular grains from the standpoint of processability and surface roughness. More specifically, the the crystal grain diameter (average) should preferably be 0.30 xcexcm or below, more preferably less than 0.30 xcexcm and, most preferably, 0.05 xcexcm or over and less than 0.3 xcexcm.
For realizing the above described physical properties and coefficient of thermal expansion, it has been found that the combination of lithium disilicate (Li2O.SiO2) and xcex1-quartz (xcex1-SiO2) as predominant crystal phases is the best combination.
The Na2O or PbO ingredient is not substantially contained in the glass-ceramic of the invention. Na2O is an ingredient which poses problems in forming of the multi-layer film. This is because Na ions diffuse in the multi-layer film to deteriorate the properties of the film. PbO is an undesirable ingredient from the viewpoint of the environment protection. Use of these ingredients, therefore, should be avoided.
Reasons for limiting the composition range of the base glass as defined in the claims will now be described.
The SiO2 ingredient is a very important ingredient for growing lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) as predominant crystal phases by heat treating the base glass. If the amount of this ingredient is below 71%, grown crystals of the glass-ceramic becomes instable and its texture tends to become coarse. If the amount of this ingredient exceeds 81%, difficulty arises in melting and forming of the glass.
The SiO2 ingredient is a very important ingredient for growing lithium disilicate (Li2O.2SiO2) as predominant crystal phases by heat treating the base glass. If the amount of this ingredient is below 8%, difficulty arises in growing of this crystal phase and also in melting of the base glass. If the amount of this ingredient exceeds 11%, the grown crystal is instable and its texture tends to become coarse and its chemical durability is deteriorated.
The K2O ingredient improves the melting property of the glass and prevents the grown crystal from becoming too coarse. The amount of up to 3% of this ingredient will suffice.
The MgO and ZnO ingredients are ingredients which improve the melting property of the glass and prevent the grown crystals from becoming coarse and also are effective for enabling the lithium disilicate (Li2O.2SiO2), xcex1-quartz (xcex1-SiO2) and xcex1-quartz solid solution (xcex1-SiO2 solid solution) crystals to grow in the globular form. For this purpose, the amount of the MgO ingredient should preferably be 0.3% or over. The amount of the ZnO ingredient should more preferably be 0.1% or over. If the amounts of these ingredients are excessive, grown crystals become instable and their textures tend to become coarse. For this reason, the amount of the MgO ingredient should preferably be 2% or less and, more preferably, 1% or less. Likewise, the amount of the ZnO ingredient should preferably be 2% or less and, more preferably, be 1% or less. The sum of the MgO and ZnO ingredients should preferably be 2% or less and, more preferably, 1% or less.
The P2O5 ingredient is indispensable as a nucleating agent. If the amount of this ingredient is below 1%, growth of nucleus will become insufficient with resulting abnormal growth of crystals. If the amount of this ingredient exceeds 3%, opaque devitrification will take place in the base glass.
The ZrO2 and TiO2 ingredients are important ingredients which, in addition to the function, like the P2O5 ingredient, as nucleating agents, are effective for making the grown crystals fine, improving the mechanical strength and improving chemical durability. If the amount of the ZrO2 ingredient is below 0.5%, these effects cannot be achieved. If the amount of the ZrO2 ingredient exceeds 5% or the amount of the TiO2 ingredient exceeds 3%, difficulty arises in melting of the base glass and ZrSiO4 and the like slug are left unmelted.
The Al2O3 ingredient is effective for improving chemical durability and mechanical strength of the glass-ceramic. The type of grown crystal differs depending upon conditions of heat treatment. Having regard to various conditions of heat treatment, the amount of this ingredient should be below 10% for growing lithium disilicate (Li2O.2SiO2) and xcex1-quartz. A preferable range of this ingredient is 5.3xe2x88x928%.
The Sb2O3 ingredient is added as a refining agent in melting the base glass. If the amount of this ingredient is below 0.1%, this effect cannot be achieved. The addition of this ingredient up to 0.5% will suffice.
The SnO2 and MoO3 ingredients may be added because they have an excellent translucency in the glass state and therefore addition of these ingredients facilitate examination of materials before crystallization. It will suffice if the amount of the SnO2 ingredient up to 5% is added and the amount of the MoO3 ingredient up to 3% is added.
The NiO, CoO, Cr2O3 ingredients may be added for adjusting the above described characteristics of the glass-ceramic within a range not impairing these characteristics. It will suffice if the amount of the NiO ingredient up to 2%, the amount of the CoO ingredient up to 3% and the amount of the Cr2O3 ingredient up to 3% are added respectively.
Additionally, the glass-ceramic of the invention is required to be free from defects such as crystal anisotropy, foreign matters and impurities and have a fine and uniform texture and further is required to have mechanical strength and high Young""s modulus in processing the glass-ceramic to small chips. The glass-ceramic of the present invention satisfies all these requirements.
For manufacturing the glass-ceramic substrate for an information storage medium according to the invention, glass materials of the above described composition are melted and is subjected to a hot or cold forming process. The formed glass is subjected to heat treatment under a temperature within a range from 550xc2x0 C. to 650xc2x0 C. for one to twelve hours for nucleation and then is subjected to further heat treatment under a temperature within a range from 680xc2x0 C. to 800xc2x0 C. for one to twelve hours for crystallization.
Predominant crystal phases of the glass-ceramic obtained by the heat treatments are lithium disilicate (Li2O.2SiO2) and xcex1-quartz (xcex1-SiO2) having globular crystal grains with a grain diameter of 0.05 xcexcm or over and 0.30 xcexcm or below.